//
// Created by daiyizheng on 2022/3/27.
//
#include <iostream>
using namespace std;
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};
//后续遍历
int countNodes(TreeNode* root) {
    if(!root)return 0;
    int left = countNodes(root->left);
    int right = countNodes(root->right);

    return left+right+1;
}
//O(n)
int countNodes1(TreeNode* root) {

}